Just a suggestion, but I'd think Shelgarn's persistant blade should have at least 3 rounds duration/level just like the summons. As it is, half the time I cast the spell it doesn't make it to my foe to attack once even before unsummoning.

At wizard level 4 I am actually very surprised how long does it last - 2 or 3 fights if it is not destroyed before. I often find myself being pursued by a Shelgarn's blade I don't need anymore.

I would like to see some immunities added though, if possible. Seeing a blade fall asleep or catch disease is mildly disturbing.

Give it construct properties and it's solved